function add_slug_class_to_li( $output, $args ) {
        $regex = '/<li class="cat-item cat-item-([\d]+)[^"]*">/';
        $taxonomy = isset( $args['taxonomy'] ) && taxonomy_exists( $args['taxonomy'] ) ? $args['taxonomy'] : 'category';

        preg_match_all( $regex, $output, $m );

        if ( ! empty( $m[1] ) ) {
            $replace = array();
            foreach ( $m[1] as $term_id ) {
                $term = get_term( $term_id, $taxonomy );
                if ( $term && ! is_wp_error( $term ) ) {
                    $replace['/<li class="cat-item cat-item-' . $term_id . '("| )/'] = '<li class="cat-item cat-item-' . $term_id . ' cat-item-' . esc_attr( $term->slug ) . '$1';
                }
            }
            $output = preg_replace( array_keys( $replace ), $replace, $output );
        }
        return $output;
    }
    add_filter( 'wp_list_categories', 'add_slug_class_to_li', 10, 2 );